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To make the cake: Position an oven rack on the middle rung.
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Heat the oven to 350 degrees (180C).
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Grease and flour the bottom and sides of two 9-by-2-inch (75-by-5cm) round cake pans, tapping out the excess flour.
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(I like to line the bottoms of the pans with parchment rounds to ensure easy and clean removal, but its not mandatory for this recipe.)
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In a medium bowl, combine the flour, baking powder, salt, and baking soda.
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Whisk until well blended.
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In a large bowl, beat the butter with an electric mixer (stand mixer fitted with paddle attachment or handheld electric mixer) on medium-high speed until smooth.
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Add the sugar and beat until well combined.
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Add the bananas and vanilla and beat until well blended and only small bits of banana remain.
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Add the eggs two at a time, beating well after each addition.
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The mixture will look curdled and a bit lumpy.
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Dont worry, it will all come together.
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Add half of the flour mixture and mix on low speed just until blended.
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Add the buttermilk and mix just until blended.
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Add the remaining flour mixture and mix just until blended.
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Stir in the walnuts, if using.
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Scrape the batter into the prepared pans, dividing it evenly.
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Bake until the tops are light brown and a toothpick or cake tester inserted in the center of 1 layer comes out clean, about 30 minutes.
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Transfer the pans to racks and let cool for about 15 minutes.
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Run a thin knife around the sides of each pan to loosen the cake.
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Invert the layers onto the racks, lift off the pans, and let cool completely.
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To make the Fudgy Frosting: While the cake is baking, make the fudgy frosting.
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Melt the unsweetened chocolate in a double boiler or in the microwave (see Notes).
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Meanwhile, combine the sugar, evaporated milk, butter, vanilla, and salt in a blender; theres no need to blend at this point.
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(For those of us who have small blendersmine is an ancient Hamilton Beach that Chris used for mixing up margaritas at collegeits impossible to make a full batch of this frosting.
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If youre not confident of the size and ability of your blender, divide the recipe in half and make two batches.)
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When the chocolate is melted, remove it from the heat or the microwave and give it a stir.
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Scrape the hot melted chocolate into the blender.
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Cover with the lid and blend on high speed until the mixture darkens and is very thick, about 2 minutes.
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Youll also hear the engine working harder then the frosting is sufficiently thick, and it will appear to be barely moving in the blender.
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It will not be pourable.
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Scrape the frosting into a clean bowl and set aside at room temperature.
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When the frosting is cool, cover the bowl with plastic wrap until the cake is completely cool and ready to frost.
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To make the Tangy Vanilla Frosting: In a large bowl, beat the butter with an electric mixer (stand mixer fitted with the paddle attachment or handheld mixer) on medium-high speed until very smooth and creamy.
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Add the confectioners sugar, vanilla, and salt and beat until blended and fluffy.
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Add the sour cream and, using a rubber spatula, gently stir just until blended.
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Cover and set aside at room temperature until the layers are completely cool and ready to be frosted.
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To frost the cake: Brush away any loose crumbs from the cooled cake layers.
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Center 1 layer, top side down, on a flat serving plate.
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To protect the plate from smears during frosting, slide small strips of foil or parchment under the bottom of the cake to cover the plate.
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Using a metal spatula or the dull edge of a table knife, spread 1 cup of the frosting evenly over the layer.
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Place the second layer, top side down, on top of the frosting.
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Be sure the sides are lined up and then press gently on the layer.
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Apply a very thin layer of frosting over the entire cake to seal in any crumbs.
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Spread the remaining frosting over the top and sides of the cake.
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Garnish the top with the coconut or the walnuts, if using.
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The cake is best served at room temperature.
